I cant get it. If you are going to use differential steering, then why don't you use 4 wheels? Usually people choose 2 wheels + caster combination because by doing so you don't need to worry about differential steering. If you place the 4 wheels, the 2 motors and the differential gear set with skill it will not only improve your control while steering but it will make your "robot" look better.

if i were going to go with the 4 wheel design, but still with differental steering. Could I just hook up a gear to the motor and and a gear on the back wheel, plus on the end of the wheel shaft have a sprocket and a chain that connects the front tire.

I have built many drive train styles using Lego. It all depends on what kind of surface you are going to drive the robot. The car style robot (one motor to power the wheels and another to steer) is not very maneuverable sometimes. And the differential steering may take many forms:- tank treads- 4 wheels like a BobCat- 2 wheels plus one or 2 castersThe differential steering is a lot faster in maneuvering but needs skills to do it right.Here are some samples:DiffSteeringCarMiniSumoRobots